A Friedel-Crafts reaction has been conventionally used as a nuclear substitution reaction of an aromatic compound.
H. O. House "Modern Synthetic Reactions" 2nd Ed. W. A. Benjamin, California (1972) pp 734-816 discloses that the reaction in which an aromatic compound is allowed to react with halogenated alkyl compounds, olefins, acid halides, acid anhydrides, esters, ethers, and cyclic ethers in the presence of Lewis acid-based catalysts such as anhydrous aluminum chloride, ferric chloride, boron trifluoride, zinc chloride, titanium tetrachloride, stannic chloride, magnesium chloride, gallium chloride, and aluminum bromide, etc., whereby an alkyl group, an acyl group, and an alkoxy group, etc. can be introduced into the aromatic compound.
In a Friedel-Crafts reaction, halogenated hydrocarbons may be generally employed or not as a reaction solvent.
Also, a novel catalyst has been proposed, for example, Japanese Patent Unexamined Publication (Kokai) No. 320089/1993 discloses a process for acylation of an aromatic compound using a Friedel-Crafts reaction in which a rare-earth elements-based Lewis acid represented by formula RE(OSO.sub.2 Rf).sub.3 [wherein RE is a rare-earth element, and Rf is a perfluoroalkyl group or perfluoroalkoxy group ] is employed as a catalyst.
As catalysts to be employed in a Friedel-Crafts reaction, its diversity has been being pursued without being satisfied with catalysts known in the present time from a viewpoint of activity, treatments after reaction, recovery and regeneration, reaction solvents suitable for the catalysts, scopes of applicable reaction, price, and methods for preparing, etc., and further more preferred new catalysts are being expected in the existing circumstances.
As a result of an intensive investigation by the present inventor, it was found that a specified hafnium compound is novel, and it is useful as a catalyst for a Friedel-Crafts reaction, and the present invention was accomplished.